1. Field of the Invention
This invention relates to the recovery of energy from geothermal brines and other hot water sources. The invention is particularly concerned with a process and system of the above type employing direct contact heat transfer between the hot brine or other hot water feed and an immiscible liquid, particularly a liquid hydrocarbon of relatively low vapor pressure, to cause boiling thereof, and effecting direct contact heat transfer between the vapor of such immiscible liquid and water to condense such vapor and heat the water, employing an arrangement of staged evaporator-condenser units for carrying out such boiling and condensation operations, and utilizing the heated water in an expander or turbine to produce work.
2. Prior Art
One of the major problems in various approaches suggested for utilization of geothermal energy is the formation of scale on the heat transfer surfaces in contact with geothermal brine. To avoid such problems flash evaporation of brine to generate steam and subsequent use of the steam for power generation or other applications have been proposed. However, in this approach a large drop of temperature will occur due to flashing and a large fraction of the total heat is lost in the brine rejected. If multi-stage flashing is performed, then the pressure of the steam in the latter stages is generally not high enough to drive a turbine.
These approaches are undesirable so far as efficient utilization of geothermal heat is concerned. To circumvent these problems multi-stage flashing of geothermal brine followed by condensation of the steam to transfer heat by indirect heat exchange to a different liquid, usually an organic liquid, has been suggested. However, the main disadvantage in the above approach is that a large surface area is still needed for heat transfer.
U.S. Pat. No. 3,988,895 to Sheinbaum discloses power generation from hot brines by passing such brine through a heat exchanger in direct contact heat exchange relation with a working fluid such as isobutane. The working fluid is vaporized and the vaporized working fluid is passed through a power extracting gas expansion device.
U.S. applications Ser. Nos. 589,068 and 611,310, both filed June 23, 1975, by S. Woinsky, disclose recovery of energy from geothermal brines by introducing the geothermal brine in a heat transfer zone in direct heat exchange relation with an organic working fluid, the heat transfer zone maintained above, at or below the critical pressure of the working fluid, and expanding the heated working fluid in an expander to produce work.
In the above prior art it is noted that the heated organic liquid is the working fluid and is employed directly as the power fluid for expansion to generate energy.
It is an object of the present invention to provide an improved process and system for recovery of energy from geothermal brines or other hot aqueous solutions of a non-volatile solute, employing direct contact heat exchange, and thereby avoiding use of metallic heat transfer surfaces. A further object is the provision of a process and system of the foregoing type, employing liquid-liquid direct contact heat exchange between an immiscible liquid and a hot aqueous solution of a non-volatile solute such as hot geothermal brine, to vaporize such immiscible liquid, and passage of such hot vapor into direct contact with a third liquid to transfer heat thereto, and utilizing the resulting third liquid for extracting energy therefrom. A particular object is to provide a process and system of the aforementioned type having high heat transfer coefficients due to intimate mixing between the brine and immiscible liquid phases, to promote boiling at low temperature differential between hot brine and immiscible liquid or hydrocarbon, and hence improved thermal efficiency.